Displays (display devices) including a display panel such as a liquid crystal panel are indispensable in various applications such as business applications and common household use. Various displays including small displays such as mobile phones, game machines, and the like and large displays such as TVs, outdoor information displays, and the like have been proposed and put to practical use.
In such displays, not only display performance, but also interactivity is highly appreciated in recent years. Based on such a trend, a touch panel is provided in front of a display screen in some displays. Further, since a liquid crystal panel includes a thin glass substrate, a glass or transparent plastic plate is provided in front of a display screen in some displays depending on applications thereof, in view of protection of a display surface and prevention of cracking of a large display. In some cases, since a hard and flat surface with a gloss is desired in view of design, a glass plate is provided in front of a display screen.
The plate such as a glass plate, a plastic plate, and a touch panel (hereinafter, referred to as a front panel) to be provided in front of a display screen is commonly disposed in front of a display surface, namely disposed in front of a surface with a polarizer attached thereto, with a space therebetween. Such disposition with a space however leads to surface reflection caused by an interface of refractive index among a display-side (inside) face of a front panel, surfaces of a polarizer, and air. Such surface reflection problematically causes phenomena of double images and image reflection which disturb displayed images. Such phenomena are caused by, as mentioned above, a refractive index difference between air and the material of a front panel. Accordingly, problems of such phenomena can be solved by providing an antireflection coating on each surface or replacing an air layer with a material having a refractive index closer to the refractive index of equipment such as a front panel and a polarizer. For example, a case where a space formed by a front panel in a liquid crystal panel is filled with a resin is disclosed (see Patent Literature 1 and the like). A case where a space formed by a front panel in a plasma panel is bonded by a resin is also disclosed (see Patent Literature 2 and the like).
A conventional display panel equipped with a front plate is disclosed in which the elastic modulus of a resin after curing is set to a predetermined value or less to overcome unevenness occurring on displayed images due to stress derived from shrinkage generated on curing of the resin that bonds a front plate (see Patent Literature 3). A liquid crystal panel equipped with a front plate is disclosed in which a guide is formed to serve as a dam around a resin to be injected between a front plate and a liquid crystal panel, and the dam is filled with a photocurable resin so that the front panel is attached to the liquid crystal panel (see Patent Literature 4). A display device is also disclosed which includes a liquid crystal display element in which liquid crystals are held between a counter substrate and a transparent substrate, an optical film provided on the transparent substrate of the liquid crystal display elements and having smaller outside dimensions than the transparent substrate, and a transparent plate bonded to the optical film and the surface of the transparent substrate around the optical film with a photocurable transparent adhesive (see Patent Literature 5). Also disclosed is a method for producing a display device including a liquid crystal display element in which liquid crystals are enclosed between a counter substrate and a transparent substrate, and a transparent touch switch, the method having the steps of applying a transparent adhesive to the transparent touch switch or the display surface of the liquid crystal display element and the like (see Patent Literature 6).
With regard to an adhesive resin, an optical stack including an antireflection film, a polarizing film, an adhesive layer, a glass cell for liquid crystal display, an adhesive layer, and a polarizing film sequentially stacked is disclosed, in which maximum values A and B of the loss tangent (tan δ) of the two adhesive layers has a ratio (B/A) of 1.1 or more (see Patent Literature 7). The maximum value of the loss tangent herein refers to, in measurement of a dynamic storage elastic modulus (E′) and dynamic loss elastic modulus (E″) by heating the corresponding adhesive layer in a temperature range of −70 to 200° C. at a rate of temperature rise of 4° C./min and at a frequency of 1 Hz, the maximum value of E″/E′ (=tan δ) within the above temperature range.
Also disclosed is an optical resin composition containing (A) acrylic acid derivative polymers, (B) acrylic acid derivatives and (C) a crosslinking agent, and having an adhesive force after curing of 3N/25 mm or more at 60° C., and tan δ of 0.40 or more at 60° C. (see Patent Literature 8).    Patent Literature 1: JP-A 2005-55641    Patent Literature 2: JP-A 2007-94191    Patent Literature 3: JP-A 2008-282000    Patent Literature 4: JP-A 2008-129159    Patent Literature 5: Japanese Patent No. 3842347    Patent Literature 6: Japanese Patent No. 3220403    Patent Literature 7: JP-A 2006-113574    Patent Literature 8: JP-A 2009-294360